This invention pertains primarily to transfer mask blanks and transfer masks employed in microlithography apparatus and methods in which a charged particle beam of electrons, ions, or the like is used to transfer a pattern, defined on the transfer mask, to a suitable substrate such as a semiconductor wafer. Microlithography is used in the fabrication of, e.g., semiconductor integrated circuits and displays.
For many years, optical microlithography (microlithography employing visible or ultraviolet light) has been the standard pattern transfer technology used in the manufacture of semiconductor integrated circuits. However, the resolution of optical microlithography systems is limited by the diffraction of light. In recent years the progressively decreasing size and increasing device density of integrated circuits has led to intensive efforts to develop a practical alternative microlithography apparatus that employs a beam of charged particles (e.g., electrons, ions, etc., hereinafter termed a xe2x80x9ccharged particle beamxe2x80x9d) or a beam of X-rays to transfer a pattern from a transfer mask (or reticle) to a suitable substrate. Charged-particle-beam (CPB) microlithography systems offer prospects for better resolution, as compared to optical microlithography systems. Current CPB microlithography systems employ a scanning electron beam for exposing a pattern onto a substrate by xe2x80x9cwritingxe2x80x9d the pattern feature-by-feature. By focusing the electron beam to a spot diameter of a few nanometers, such a system can form very fine pattern features, sized 1 xcexcm or smaller.
However, conventional electron-beam exposure systems write only one line at a time, and the finer the pattern the more xe2x80x9cfocusedxe2x80x9d the electron beam must be for drawing (i.e., the smaller the xe2x80x9cspot diameter,xe2x80x9d or area illuminated by the beam at any instant). Hence, drawing time is increased and throughput correspondingly decreased. From the perspective of device production costs, electron-beam exposure systems that write the pattern feature-by-feature cannot be used practicably to expose wafers for mass production.
To increase throughput over that obtainable using CPB xe2x80x9cwritingxe2x80x9d systems, CPB projection-transfer apparatus have been proposed. In such apparatus, an electron beam (as a representative charged particle beam) illuminates all or a portion of a pattern defined on a transfer mask. An image of the illuminated region of the transfer mask is demagnified as the image is projected onto a corresponding region of the wafer. Such projection is performed by passing the beam, after propagating through the transfer mask, through a CPB projection-optical system (projection lens) located between the transfer mask and the substrate.
For use in a xe2x80x9cdemagnifyingxe2x80x9d (or xe2x80x9creducingxe2x80x9d) CPB projection-exposure apparatus, a transfer mask (also termed a xe2x80x9creticlexe2x80x9d) is required upon which a circuit pattern is drawn (i.e., the mask xe2x80x9cdefinesxe2x80x9d the circuit pattern). Referring to FIGS. 3(a)-3(b), types of transfer masks currently used with such apparatus include: (1) scattering-transmission masks 11 (FIG. 3(a)), in which pattern features (or pattern xe2x80x9celementsxe2x80x9d) are defined by corresponding CPB-non-scattering regions 15 of a CPB-transmissive membrane 12; and (2) scattering-stencil masks 21 (FIG. 3(b)) in which pattern features are defined by a corresponding pattern of through-holes 24 defined in a membrane 22. The membrane 22 in the scattering-stencil mask of FIG. 3(b) is sufficiently thick to scatter an electron beam, whereas the membrane 12 in the scattering-transmission mask of FIG. 3(a) transmits the electron beam with little to no scattering.
Each of the masks of FIG. 3(a) and FIG. 3(b) is typically divided into multiple small regions 12a and 22a, respectively, that are individually exposed. Each small region 12a, 22a is also termed an xe2x80x9cexposure unitxe2x80x9d or xe2x80x9csubfieldxe2x80x9d in the art. Each exposure unit 12a, 22a defines a respective arrangement of pattern features or elements (corresponding to the respective portion of the overall pattern defined by the particular exposure unit) to be projection-transferred to a sensitive substrate. The exposure units 12a, 22a are divided from one another on the mask by boundary regions 13a, 23a, respectively, in which no pattern features are defined. The boundary regions 13a, 23a are typically the regions of the mask in which supporting struts 13 and 23, respectively, are located.
In a scattering-stencil mask 21 (FIG. 3(b)), the membrane 22 consists of a silicon membrane, about 2 xcexcm thick, that defines apertures 24 for passage of the electron beam. The pattern of apertures 24 in the membrane 22 of an exposure unit 22a defines the respective portion of the pattern to be transferred to the sensitive substrate (normally situated downstream of the mask).
In either of the masks (FIGS. 3(a)-3(b)), the exposure units 12a, 22a are typically small, each measuring, for example, about 1 mm square (i.e., 1 mmxc3x971 mm). At any one instant, only one exposure unit 12a, 22a is illuminated by the electron beam, and the exposure units of the pattern are typically illuminated in an ordered manner (e.g., sequentially). Each exposure unit 12a, 22a defines a corresponding portion of the overall pattern defined by the mask for transfer to a chip region (xe2x80x9cdiexe2x80x9d) on the sensitive substrate. The entire pattern to be exposed onto each die is defined on the respective mask 11, 21 by a large number of exposure units 12a, 22a arrayed over a large area of the mask.
Either of the masks 11, 21 can be used for projection exposure as shown in FIG. 4. A charged particle beam exposes each exposure unit 12a (22a) in a sequential manner. The respective pattern portion corresponding to the respective arrangement of apertures (or non-scattering membrane regions) in each exposure unit 12a (22a) is demagnified (reduced) and transferred to the sensitive substrate 17 by a projection-optical system (not shown). The images of the various exposure units 12a (22a) are positioned relative to each other so as to be xe2x80x9cstitchedxe2x80x9d together on the sensitive substrate 17. As can be seen in FIG. 4, xe2x80x9cdemagnificationxe2x80x9d results in an image on the substrate 17 that is smaller than the corresponding region on the mask.
Conventional transfer mask blanks and transfer masks are manufactured by a process as illustrated in FIGS. 5(a)-5(f). A silicon substrate 1, having a (100) crystal-lattice orientation on its surface, is prepared or otherwise provided (FIG. 5(a)). Boron is diffused into the silicon on one side of the substrate 1, either by thermal diffusion or by ion implantation, forming an xe2x80x9cactivation layerxe2x80x9d 2. The activation layer 2 is destined to serve as an etch-stop layer during a later etching step (desirably wet, or anisotropic, etching), and ultimately becomes the silicon membrane of the mask. Regions of the silicon substrate 1, other than the silicon activation layer 2, are known as the xe2x80x9csupport siliconxe2x80x9d 1a (FIG. 5(b)).
Next, a thin film of silicon nitride 3 is formed over the entire surface of silicon substrate 1 (FIG. 5(c)). Selected regions of the silicon nitride film 3 formed on the rear surface of the substrate 1 are removed to form windows 4. The remaining silicon nitride film 3 forms a xe2x80x9cwet-etchxe2x80x9d mask 5, for use in a subsequent wet-etching step (FIG. 5(d)).
FIG. 5(d) shows a window 4 in one location. But, in actual practice, a large number of windows 4 are formed as required to define corresponding exposure units on the mask.
After forming the wet-etch mask, the entire silicon substrate 1, with wet-etch mask 5 formed thereon, is immersed in an etching solution such as a KOH solution. In the etching solution, anisotropic etching of the support silicon 1a proceeds depthwise into the thickness dimension of the silicon substrate 1 from each window 4 (the windows are regions not protected from etching by the silicon nitride film). When etching reaches the activation layer 2, the depthwise etching rate decreases steeply, so that depthwise wet-etching effectively halts at the activation layer 2 (FIG. 5(e)).
Proper functioning of an activation layer 2 as an etch-stop layer depends upon the boron concentration in the silicon of the activation layer 2. If the boron concentration is less than 2xc3x971019 atoms/cm3 in the activation layer, then the rate at which depthwise etching decreases at the activation layer is insufficient to adequately prevent etching into the support silicon 1a. Therefore, the boron concentration in the activation layer 2 is desirably at least 2xc3x971019 atoms/cm3.
After completing wet etching the silicon nitride mask 5 is removed, thereby completing formation of the transfer mask blank (FIG. 5(f)). The completed transfer mask blank consists of the silicon membrane 2a (i.e., the boron-doped silicon activation layer), and silicon struts 1b (which are the remaining portions of the support silicon 1a that were not etched away by the etching solution. The struts 1b thus effectively support the membrane 2a. 
To create a stencil-transfer mask from the mask blank shown in FIG. 5(f), the following procedure is typically used:
A layer of resist is applied to the membrane 2a of the transfer mask blank, and the specified mask pattern is transferred to the resist using, e.g., an electron-beam pattern-drawing apparatus. Unexposed regions of the resist are removed, and the corresponding unprotected regions of the membrane 2a are then etched through. Thus, the specified pattern is transferred to the transfer blank, completing formation of the stencil-transfer mask.
A conventional transfer mask blank made according to the previously described manufacturing process (FIGS. 5(a)-5(f)) typically exhibits distortion of the membrane 2a whenever the specified pattern is formed on the membrane 2a. Consequently, the mask pattern is correspondingly deformed.
From studies to determine the cause of membrane distortion, the inventor found that a non-uniform distribution (i.e., variation) of boron concentration exists depthwise in the thickness dimension of a conventional mask. The variation of boron concentration was found to be linked to membrane distortion, and thus to pattern deformation in the finished mask.
FIG. 6 shows a representative variation of boron concentration depthwise in the thickness dimension of a membrane in a conventional transfer mask blank. The boron concentration is plotted on the Y-axis (ordinate), and depth in the thickness dimension of the membrane is plotted on the X-axis (abscissa), with the membrane""s outermost surface (the surface at which boron-doping occurred) at X=0. The variation of boron concentration was measured by SIMS (Secondary Ion Mass Spectrometry). This graph of FIG. 6 shows that the boron concentration is highest slightly below the membrane""s outermost surface, and that the boron concentration decreases with depth from that surface. The variation in the concentration of boron (xe2x80x9c[B]xe2x80x9d) depthwise through the thickness dimension of the membrane is about 30%, calculated as follows:
([Bmax]xe2x88x92[Bmin])/[Bavg]
wherein [Bmax] is the maximal boron concentration, [Bmin] is the minimum boron concentration in the membrane, and [Bavg] is the average (mean) depthwise concentration of boron. In this example, [Bavg] is 1.6xc3x971020 atoms/cm3.
The inventor also found that tensile stress of the membrane is greatest at or near the membrane surface where the boron concentration is highest. Tensile stress is lower at certain depths (in the thickness dimension) inside the membrane. Hence, after forming a mask pattern on the membrane, the stress curve is convex toward the membrane surface, which causes the membrane (and thus the mask pattern) to become distorted.
In order for an activation layer 2 to function satisfactorily as an etch-stop layer during KOH wet-etching, the boron concentration in the activation layer should be at least 2xc3x971019 boron atoms/cm3. However, as the inventor has discovered, this does not necessarily imply that the higher the boron concentration the better. As noted above, local tensile stress in the membrane 2a increases with increases in the local boron concentration. Consequently, whenever a pattern is formed on the membrane of a transfer mask blank (in which the silicon activation layer has a high boron concentration), excessive pattern deformation occurs, which is problematic.
Compared to transfer masks used in conventional photolithography, transfer masks used for projection-exposure using a charged particle beam or an X-ray beam require extraordinary accuracy and precision in pattern definition and positioning. Such accuracy and precision is on the order of tens of nanometers, due to the extremely small size of individual elements (features) in integrated circuits manufactured using such methods. These small feature sizes make the problem of distortion of the mask membrane critical to solve.
In view of the foregoing, one object of the present invention is to provide transfer mask blanks in which tensile stress is substantially reduced, compared to conventional transfer mask blanks. Such mask blanks can be used to form transfer masks that exhibit substantially no pattern deformation. Optimal ranges of certain respective parameters have been found that result in little to no membrane distortion (and thus little to no pattern deformation) in the completed mask.
From the standpoint of reducing tensile stress as much as possible in the mask blank, the boron concentration in the silicon membrane is desirably in a range of 2xc3x971019 to 5xc3x971020 atoms/cm3. Furthermore, the variation in boron concentration is desirably no more than 10 percent throughout the membrane, including through the thickness dimension of the membrane. A transfer mask made from such a mask blank exhibits substantially no membrane distortion (and thus no pattern deformation).
Hence, a transfer mask blank according to the present invention comprises a silicon membrane (on which a pattern, to be transferred to a sensitive substrate, will be formed when the mask blank is made into a mask). The silicon membrane is doped with boron at a concentration in the silicon membrane of 2xc3x971019 to 5xc3x971020 atoms/cm3. Also, the variation in the boron concentration throughout the silicon membrane is no more than (i.e., xe2x89xa6) 10%. A transfer mask according to the invention comprises a silicon membrane upon which a pattern to be transferred to a sensitive substrate is formed, or that defines such a pattern. The silicon membrane is doped with boron at a concentration in a range of 2xc3x971019 to 5xc3x971020 atoms/cm3. The variation in the boron concentration throughout the silicon membrane is no more than (i.e., xe2x89xa6) 10%.
The foregoing and additional features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.